Standard four-bar parallelogram linkages are widely used. These devices have limited angular range capacity and their stiffness varies with angle. In such linkages, the links will deform in use. This deformation introduces linkage resonances which is a disadvantage when using such linkages in high performance servo control loops.
Also, conventional four-bar parallelogram linkages are limited in use to less than ninety degrees rotation because at the ninety degree point they can invert and lose their proper angular orientation. This is a serious problem in control mechanism,s particularly in an arrangement such as a high performance servo control loop.
A solution to these problems has been sought which would provide a linkage with large angle capability and with constant stiffness.